1. Field of the Invention
The present invention relates to a semiconductor integrated circuit and, more particularly, to a memory embedded logic integrated circuit mounting memory circuits having different performances on the same semiconductor chip.
2. Description of the Related Art
Recently, the importance of semiconductor integrated circuits called system LSIs is increasing. Conventionally, semiconductor chips are separated in units of functions. An LSI in which most of these functions are integrated in one chip is called a system LSI. Of these system LSIs, memory embedded logic LSIs in which a memory circuit and logic circuit are integrated in one chip have been particularly extensively developed in recent years.
One great merit of integrating a memory circuit and logic circuit in one chip is low power consumption. Since a memory circuit and logic circuit are integrated in one chip, no I/O buffers need be formed between these two circuits. This reduces the power consumption required for charge/discharge by these I/O buffers.
In addition, since the bus width can be increased, the data transfer rate can be raised even if the operating frequency is lowered. This also reduces the power consumption.
For example, when in digital apparatuses for consumers a memory is formed in a different chip as in the conventional devices, the power consumption cannot be lower to 1 W or less. Accordingly, expensive packages and heat sinks are required to increase the heat radiation characteristic. This extraordinarily increases the fabrication cost. Embedding a memory in the same chip is necessary to use conventional inexpensive plastic packages.
Another merit of integrating a memory circuit and logic circuit in one chip is a high operating speed. The bus width of a general-purpose memory is at most about 32 bits, and its operating frequency is about 200 MHz. Hence, the data transfer rate cannot be greatly increased.
In a memory embedded logic LSI, the bus width can be readily increased to about 256 bits. Accordingly, the data transfer rate can be made much higher than that of the general-purpose memory even at the same operating frequency as that of the general-purpose memory.
Of memory embedded logic LSIs, particularly DRAM (Dynamic Random Access Memory) embedded logic LSIs have been put into practical use since around 1996 for high-performance applications such as image processing in engineering workstations and in high-performance personal computers. Embedding a DRAM in the same chip increases the band width of a memory bus and can greatly increase the processability.
Recently, DRAM embedded logic LSIs are beginning to be extensively used in image apparatuses such as digital video cameras and still cameras, storage media such as a DVD (Digital Video Disk), and portable information terminals. In these apparatuses, the use of DRAM embedded logic LSIs enhances the effect of reducing the power consumption and the mounting area.
Since system LSIs are used in various applications as described above, DRAMs to be mounted on these system LSIs are also required to have various performances in accordance with these applications. For example, in dry-cell-driven digital apparatuses for consumers, low power consumption is required more than a high operating speed. In image processing, a large memory scale is necessary. In recent three-dimensional image processing and signal processing for communication systems, demands for high-speed access are increasing. In the future, development may progress to integrate a system of a much larger scale on one chip.
In a case like this, memory circuits formed in the same chip may be required to have one memory circuit in which the integration degree is given high priority and another memory circuit in which the operating speed is given high priority. For example, two DRAM circuits integrated in a system LSI are required to have different performances; one is required to have a high access speed, and the other is required to give preference to the data holding characteristic and the integration degree rather than to the access time.
DRAM circuits mounted on conventional DRAM embedded logic LSIs are so fabricated as to have performances meeting as many product requirements as possible. Therefore, the memory capacity, data bus width, operating frequency, read latency, address assignment method, and bank configuration can be changed in accordance with the product specification in many cases.
However, these DRAM circuits cannot be adapted to changes in the performance of a memory cell array itself. That is, memory cells used in two types of DRAM circuits integrated on the same chip have the same performance.
This expectedly makes it difficult to meet product requirements to be more and more diversified in the future.
As described above, in the conventional semiconductor integrated circuit in which a plurality of different types of memory circuits are integrated in the same chip, the performances of memory cells used in these memory circuits are the same. This makes diverse product requirements difficult to satisfy.